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Exoplanet Missions Beyond Kepler

Because it’s going to be an interesting week for exoplanet studies (for reasons I’ll talk about soon, though not today), I’ll lead off with some thoughts on eta-Earth, defined as the fraction of Sun-like stars with a planet like Earth orbiting them. We have a lot to learn about the frequency of terrestrial worlds, and as Philip Horzempa points out in a recent article for The Space Review, the image that’s gradually emerging is of fewer ‘Earths’ than Carl Sagan once estimated when he said in the 1980s that half of all stars could have a planet like our own.

Image: Artists’ concepts of small exoplanets compared to our own planets Mars and Earth. As Kepler continues to hunt, how can we move beyond its findings to learn more about terrestrial planets around much closer stars? Credit: NASA/JPL-Caltech

With Kepler’s continuing datastream and improving ground-based instrumentation, we’re learning more about planet distribution, but Horzempa notes that even now, estimates of Earth analogs have ranged from about 2 percent to as high as 35 percent. He cites John Rehling, who went to work on biases inherent in the Kepler data set, namely that 1) the Kepler data are more complete in regions close to the host star and 2) the transit method detects larger planets more readily than small ones (see Looking Into Kepler’s Latest for more on this analysis, which appeared last March). Rehling’s findings led him to support a low 2% figure for eta-Earth but more complete Kepler data now has him backing off the number. Here’s what Horzempa says on the matter:

[Rehling’s] analysis has shown that the abundance curves for certain planet types, such as super-Earths, do not follow a smooth distribution curve. Rather, there is a peak, followed by a falloff, with increasing distance from the parent star. The distribution curve for Earth-sized worlds shows a positive slope, so far. Therefore, at present the best that can be said about eta-Earth is that it probably has a value of 2–12%. Only an extended Kepler mission will be able to determine the actual fraction of stars that have an Earth orbiting it. A low value for eta-Earth will mean that the search for Earth analogs in nearby solar systems will need to be pursued with vigor, and with multiple approaches.

Kepler, of course, is working with a large field of stars about 2000 light years away, the idea being to gather statistical information that will help us understand the broad trends of planet distribution. The mission has been extraordinarily successful, but it’s interesting to speculate, as Horzempa does, on what missions could now follow up on its findings. The article looks at the Gaia space telescope that should be operational by 2014 as well as the New Worlds sunshade concept that could be deployed for use with the James Webb Space Telescope, but I want to focus on Horzempa’s third mission concept, an ‘Earth’ finder called NEAT.

The Nearby Earth Astrometric Telescope has been proposed to the European Space Agency as a mission that could home in on nearby terrestrial planets in ways Kepler cannot. The idea here is to fly a pair of spacecraft separated by some 40 meters, providing the needed focal length to create high angular resolution. One of the spacecraft carries a 1-meter mirror while the other is a detector probe that collects the focused light onto an array of CCDs. The goal is to detect Earth analogs within 50 light years, with an accuracy of 0.05 microarcseconds for selected targets. Says Horzempa:

The NEAT duo will observe a list of 200 nearby Sun-like stars over several years. It will be able ascertain whether planets down to a mass of 1 Earth, or larger, orbit those stars. In addition, NEAT will be able to survey the solar systems discovered by Kepler, detecting giant planets that were missed by Kepler. These would be planets that did not transit their parent star during the Kepler mission, either because of the “wrong” orbital inclination or because their orbital period is too long. This will help to “fill in” the architecture of those solar systems, allowing a better understanding of how those systems are formed.

As you can see, NEAT would require formation flying in space, something that will demand a precursor experiment that the French space agency CNES has already funded, drawing on progress in formation flying that has been achieved by Sweden’s PRISMA program. The hope of the NEAT planners is to move beyond this experiment to a smallsat follow-up called micro-NEAT, which would put the formation flying algorithms through their paces, though at a separation of 12 rather than 40 meters. Quite a lot of serious work could emerge from such a mission: Horzempa notes that micro-NEAT would be able to detect planets down to one Earth mass for our nearest neighbors, Alpha Centauri A and B. It would also be able to detect larger planets (with a lower mass limit of 10 Earth masses) around the 25 nearest stars.

You’ll want to read all of Horzempa’s essay, which will include a second installment to be published today. With funding for projects like DARWIN and Terrestrial Planet Finder in perpetual limbo, smaller missions that can advance the exoplanet hunt — and help us refine the value of eta-Earth — are the next priority. They will be designed and, let’s hope, fly in an environment enlivened with still more detailed findings from our existing space observatories as well as instruments on the ground that reveal planets in the habitable zone of their stars.

NEAT was submitted as an answer to the call for proposals for M-class space missions in ESA’s “Cosmic Vision 2015-2025” plan. For more, see Mablet et al., “High precision astrometry mission for the detection and characterization of nearby habitable planetary systems with the Nearby Earth Astrometric Telescope (NEAT),” available online. The NEAT proposal summary is also available. A NEAT workshop was held in November of 2011 in Grenoble, France.


Comments on this entry are closed.

  • andy October 15, 2012, 13:37

    Astrometry does have something of a tainted reputation in the field of exoplanet discovery (e.g. the Barnard’s Star saga) but it does seem to be one of the best ways to find nearby potentially-habitable planets. As far as I can tell it tends to be less sensitive to stellar activity than radial velocity studies.

    Certainly does seem to be an interesting time for exoplanets, what with the carbon planet hypothesis for 55 Cnc e (I quite like this idea, but certainly we are a long way from proving it despite the media hype), the first planet in a quadruple star system and of course the ESO announcement tomorrow…

  • Sutpen October 15, 2012, 15:02

    As much good as he did for science, especially the subject of this site, we have to realize that Carl Sagan fell into or perhaps never escaped seeing things through a thoroughly human lens. Consider his expectation that advanced aliens would be wise and benevolent — much the same thing Adamski claimed for the inhabitants of Mars and Venus he claimed to meet in the 1950s. Given our present knowledge the creatures in the Alien movies are as likely.
    We just have to gather information with as few assumptions as possible…

  • Mike October 15, 2012, 16:22

    A smart cheap project may accomplish significant things. One can’t help but wonder if only the micro-NEAT is what ends up being funded. Even that is maybe iffy in these tight-fisted times. There are no shortage of very good ideas for space based exoplanet finders and exolife detectors. There is ofcourse a shortage of funding (vision). The astrometric method that NEAT would employ is what is required for a comprehensive nearby survey. Get the Chinese astronomy community interested, that’s where the money is.

  • coolstar October 15, 2012, 18:25

    I’ll admit I haven’t read up on NEAT yet, but if the object is just to get a long focal length (and not do interferometry), it’s not clear that free flyers doing station keeping is even necessary. Seems as though 40 meters could be done with a telescoping boom (perhaps at one of the Lagrange points), with active readouts and/or active optics to decrease the image jitter by the necessary amount. Seems much easier from an engineering point that free flyers. Of course, it would build up the experience necessary to perhaps later do free flyers on an interferometry mission.

  • Adam October 15, 2012, 18:47

    An annoying lacuna in our knowledge is the response of an otherwise Earth-like planet to differences in insolation. Venus receives ~twice the sun as Earth and has evolved very differently. What would Earth be like at 1/2 the sunlight? Our models vary considerably. The confusion over early Mars and the Faint Young Sun widen the uncertainty – an intermittently wet Mars and a frozen early Earth or wet (and hot) conditions for both are about equally possible.

  • Enzo October 15, 2012, 19:09

    ESA might not have the same technical capabilities as NASA nor access to RTG and they are rubbish at PR. However, at least at the moment they seem to have a much more varied (and hence more scientific) exploration program instead of the Mars obsessed-at-the-expense-of -everything-else NASA.
    The cancellation of SIM and the recent appalling choice of Insight over the fantastically more interesting and similarly priced TiME just confirms it.
    I hope that NEAT is approved and SIM’s void is finally filled.

  • coolstar October 15, 2012, 23:11

    Enzo, from my point of view, the same can be said of the ESO’s ground based efforts compared to those of American institutions. Their ground based programs are much broader based and at least as successful and rigorous as American programs. They even look at STARS, god forbid, even stars that do NOT have planets known to orbit them……

  • ljk October 16, 2012, 9:42

    Future exoplanet missions: NASA and the world (part 2)

    A wide variety of spacecraft missions, both proposed and under development, can support the discovery and study of extrasolar planets. Philip Horzempa concludes his two-part look at these missions, and the need to better organize and fund exoplanet research at NASA.

    Monday, October 15, 2012


  • ljk October 16, 2012, 10:13

    andy said on October 15, 2012 at 13:37:

    “Astrometry does have something of a tainted reputation in the field of exoplanet discovery (e.g. the Barnard’s Star saga) but it does seem to be one of the best ways to find nearby potentially-habitable planets. As far as I can tell it tends to be less sensitive to stellar activity than radial velocity studies.”

    Is “tainted reputation” really the right words here? It is not like Van De Kamp deliberately tried to fool astronomers into thinking that the red dwarf had two gas giant companions.

    If anyone had the celestial rug pulled out from under them it was Van De Kamp himself, who spent decades trying to find exoplanets when most other astronomers dismissed the endeavor – until they were finally established in 1995, then of course just about everyone jumped on that bandwagon, as will be the case when extraterrestrial life is found one day.


    Even the man who disproved Van De Kamp’s findings did so with no joy and not a little reluctance.

    The guy who thought he found exoplanets around a pulsar in 1990 also learned he was mistaken for similar reasons (the perils of doing delicate astronomy from the surface of a big moving space rock with air) and was practically lauded as a hero when he admitted his error at an astronomical conference.

    So why not give the same credence to a man who spent most of his life and reputation pursuing something few else did in his time and now it is big science with multimillion dollar satellites making major discoveries about alien worlds every other day.

    And do not forget – the BIS took Van De Kamp seriously enough to aim the Daedalus probe at Barnard’s Star rather than Alpha Centauri. It would have taken the interstellar vessel fifty years to reach Barnard’s Star, as opposed to just 34 years to our nearest stellar neighbors. A rather crucial time difference when it comes to waiting for data from another solar system.

  • Ronald October 16, 2012, 11:53

    Well, as long as we are not able to routinely and automatedly detect and characterize entire planetary systems, something which would require enormous space-based telescope arrays, it would make a lot of sense to be able to do some sound *modelling* of planetary systems. By this I mean to produce reliable theoretical models that have a very high degree of predictive value on the basis of the main characteristics of the star, in particular its (relative) elementary abundance.
    This is another field which is quietly and steadily progressing nowadays.
    I am convinced that in the near future we will be able to predict, with high accuracy, what kind of planetary system a particular star will have, on the basis of its main characteristics.

  • Ron S October 16, 2012, 12:47

    coolstart: “Seems as though 40 meters could be done with a telescoping boom…”

    The NEAT summary Paul referenced mentions a boom option. Perhaps at the time it was written they were still making design decisions, and they have since chosen to use free fliers.

  • Tom Mazanec October 16, 2012, 13:54

    When I was an Astronomy major in the late 70s, I thought I would die of old age never knowing if other stars had planets. In a matter of months, we should know of over 1000 confirmed extrasolar planets. In spite of the fiscal situation at present, I am optimistic about the future.

  • Paul Gilster October 16, 2012, 14:10

    Ron S writes:

    The NEAT summary Paul referenced mentions a boom option. Perhaps at the time it was written they were still making design decisions, and they have since chosen to use free fliers.

    Yes, the problem is jitter and the boom option keeps introducing too much of it. That’s why they’re pushing free fliers.

  • andy October 16, 2012, 14:35

    Is “tainted reputation” really the right words here? It is not like Van De Kamp deliberately tried to fool astronomers into thinking that the red dwarf had two gas giant companions.

    Well the reputation it has is that astrometric planet discoveries tend to evaporate in the light of further scrutiny, this continued to be the case as recently as the 2009 claim for a planet orbiting the ultracool dwarf VB 10.

    So far the Extrasolar Planets Encyclopaedia lists one planet that was discovered by astrometry – in the binary system HR 7162, this has not yet been confirmed by an independent method (preferably something which could determine which of the two stars the planet is orbiting…)

  • Ron S October 16, 2012, 14:59

    Hmmm. If either component is moving/vibrating (jitter?) it doesn’t much matter whether there is a boom or not. I would guess that there are problems with either stored mechanical stresses or thermal stresses in the boom that release in some unpredictable fashion. Considering the precision objective it would take very little of this to push the error bars too wide.

  • Raimo Kangasniemi October 20, 2012, 0:34

    Now we have a now exoplanet mission confirmed, Cheops from ESA, launching in 2017 for a 3.5 year mission. Apparently it can’t study smaller planets than Super-Earths.

  • Eric January 7, 2013, 21:08

    “(preferably something which could determine which of the two stars the planet is orbiting…)”


    My question, relating to your quote above is this: Is it possible for a planet to be pushed and pulled around both stars of a binary system rather than having one orbit around just one of the stars?


    Eric Jackson
    Sacramento, CA

  • JIM February 7, 2013, 22:31


    It’s probable.

    I believe it is more likely that a planet in a binary star system will have a more stable orbit around both stars, or more correctly, around the center of mass of both the stars.